Autoscaling: Move target count independent of actual count.

This should let us grow and shrink the worker pool in chunks when necessary
(like when a bunch of them go offline, or when there is a worker version
change).
Fix import
NoopTask: Fix things that should be static. Add simple factory method.
Autoscaling changes from code review.

- Log and return immediately when workerSetupData is null
- Allow provisioning more nodes while other nodes are still provisioning
- Add tests for bumping up the minimum version
Autoscaling: Terminate obsolete workers faster
Merge pull request #339 from metamx/autoscaling

Autoscaling: Move target count independent of actual count.
[maven-release-plugin] prepare release druid-0.6.43
[maven-release-plugin] prepare for next development iteration
Autoscaling fixes.

- Initial targetWorkerCount must be subject to pool size limits
- Use consistent workerSetupData for the entire autoscaling run
- Don't call terminate() when we have nothing to terminate
- Terminate obsolete workers even faster
[maven-release-plugin] prepare release druid-0.6.44
[maven-release-plugin] prepare for next development iteration
Update versions
Fix RemoteTaskRunnerTest
[maven-release-plugin] prepare release druid-0.6.45
[maven-release-plugin] prepare for next development iteration
RealtimeIndexTask: Clean up imports and comments
RealtimePlumberSchool: Alert and continue on any exception when abandoning segments
Indexing service: Fix termination related log message
Update versions
[maven-release-plugin] prepare release druid-0.6.46